1. Field of the Invention
The present invention relates to a steering system for a straddle type four-wheeled all-terrain vehicle (hereinafter abbreviated to "ATV") and, more specifically, to a steering system capable of limiting the turning of a steering shaft to a predetermined angular turning range.
2. Description of the Related Art
The ATV is capable of traveling rough terrains including sandy places, rugged lands, rocky mountains and rough lands with muddy puddles. A steering shaft is supported for rotation on a body frame of the ATV, and a handlebar is connected to the upper end of the steering shaft. The driver operates the handlebar to steer the ATV. The steering shaft is linked to a wheel support system supporting wheels by a linkage. When the handlebar is operated by the driver, the direction of the wheels is changed for steering. The turning of the steering shaft of the ATV is limited to a predetermined angular turning range by a stopping mechanism similar to a known handlebar stopping mechanism employed in a motorcycle.
Referring to FIGS. 10 and 11 illustrating a handlebar turning angular turning range adjusting mechanism for a motorcycle, disclosed in JP-A No. 61-196887 (Utility Model), a handlebar 132 is fixedly joined to a lower fork bracket 121, and stoppers 122 project from the upper surface of the lower fork bracket 121. A head pipe 130 is fixed to a body frame, and a bracket 154 is fixed to the outer side surface of the head pipe 130. Stopper bolts 155 are attached to stopper bolt holders 156 fixed to the bracket 154. When the handlebar 132 is turned on the head pipe 130 by the driver, the lower fork bracket 121 is turned. The handlebar 132 cannot be turned beyond a position where the stopper 122 comes into contact with the stopper bolt 155. In the motorcycle, the handlebar stopping mechanism is disposed at an upper position near the handlebar to make the handlebar stopping mechanism rigid against force exerted thereon by the handlebar so that the driver is able to recognize the turn of the handlebar to a limit position from the unyielding collision of the stopper bolt 155 against the stopper 122. The steering shaft of the ATV is combined with a handlebar stopping mechanism similar in principle to the foregoing handlebar stopping mechanism for a motorcycle.
Referring to FIG. 12 showing a handlebar stopping mechanism employed in a conventional ATV, a handlebar bracket 231 is joined to the upper end of a steering shaft 230. A handlebar is attached to the handlebar bracket 231. A stopper 253 provided with a bolt 255 is welded to a side surface of an upper part of the steering shaft 230. An upper bracket 211 is connected to a body frame F. When the steering shaft 230 is turned to a predetermined angular position, the bolt 255 of the stopper 253 collides with the bracket 211. An upper bearing unit 212 is attached to the bracket 211 to support the steering shaft 230 for turning. The stopper 253 provided with the bolt 255 is attached to a part of the steering shaft 230 extending upward from the upper bearing unit 212. FIGS. 13A and 13B show the positional relation between the stopper 253 and the bracket 211 when the handlebar is held at a position for straight forward traveling and when the handlebar is turned substantially fully to the right, respectively. The turning of the steering shaft 230 in either direction is limited by the collision of the head of the adjusting bolt 255 with the bracket 211 to limit the turning of the steering shaft 230 to a predetermined angular turning range.
Thus, the handlebar stopping mechanism employed in the ATV, similarly to that employed in a motorcycle, is disposed at an upper position near the handlebar to make the handlebar stopping mechanism rigid against force exerted thereon by the handlebar so that the driver is able to recognize the turn of the handlebar to a limit position from the unyielding collision of the head of the bolt of the stopper against the bracket.
Sometimes intensive shocks that force the direction of the wheels to change suddenly act on the wheels because the ATV travels rough terrains. Those shocks exerted on the wheels are born through a linkage linking a wheel support mechanism and the steering shaft 230 by the stopper attached to the upper part of the steering shaft 230. The linkage is connected to a lower part of the steering shaft 230 apart from the upper part of the same. Since the stopper is distant from a position on which the shocks are exerted, the steering shaft 230 must be highly rigid and hence the steering shaft 230 is inevitably heavy.